In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices like integrated circuits (ICs) and electro-optical devices, and development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being hastened.
Various metal oxides are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxides having semiconductor characteristics is known (for example, see Patent Documents 1 to 4, Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, homologous compound, InGaO3(ZnO)m (m is natural number) has been known as a multi-component oxide containing In, Ga and Zn (for example, see Non-Patent Documents 2 to 4 and the like).
Furthermore, it has been confirmed that an oxide semiconductor including such an In—Ga—Zn—O-based oxide is applicable to a channel layer of a thin film transistor (for example, see Patent Document 5, Non-Patent Documents 5 and 6).